New products for building industry using clay, leather and glass
One of purest of the clays composed chiefly of the mineral kaolinite usually formed when granite is changed by hydrothermal metamorphism. Usage of the terms china clay and kaolin is not well defined; sometimes they are used synonymously for group of similar clays, and sometimes kaolin refers to those obtained in the United States and china clay to those that are imported. Some authorities term as clays the more plastic of the kaolins. Clays have long been used in the ceramic industry, especially in fine porcelains, because they can be easily molded, have a fine texture, and are white when fired. The clays are also used as filler in making paper. In the United States, deposits are found primarily in Georgia, North Carolina, and Pennsylvania; china clay is also mined in England and France. The pressures generated by this movement caused the seabed to fracture and fold. The folding process was accompanied by intense subterranean activity, the masses of molten rock forced their way upwards.
When the rock cooled, it became granite, a rock made of a mixture of quartz, feldspar and mica. The formation of the granite took place between 290 and 270 million years ago and today it forms the rocky backbone of the south-west of England, being exposed in the Scilly Isles, Lands End, Carnmenelis, Hensbarrow near St Austell, Bodmin Moor and Dartmoor. Granite is one of the commonest igneous rocks, but varies considerably in its composition from place to place. While the quartz is never anything but quartz, the feldspar can be a silicate of alumina with potash, soda or lime and the mica the potash-rich muscovite or the iron-rich biotite. In some parts of the South West, the feldspar in the granite is higher in its soda content than its potash content and these places are where china clay is found today. It came into being through a complex sequence of events. While the molten rock was still cooling, it was attacked successively by steam, boron, fluorine and tin vapour, these acting on the alkali content of the feldspar and converting it into china clay.
In certain locations, the deposits occur in clusters, and here one finds the largest of the clay workings, including Blackpool, Goonbarrow, Littlejohns and Melbur in the St Austell area, Stannon on Bodmin Moor and Lee Moor on Dartmoor. Some of the deposits cover many hectares at the surface, while the deepest of the clay pits currently being worked is some 130 metres deep. To work any pit beyond this depth would mean that the pit would have to be enlarged laterally, as well as in depth. The clay occurs in the deposits in the form of china clay rock, a mixture of up to 15 per cent china clay and up to 10 per cent mica, and the remainder being quartz. The clay itself varies considerably from pit to pit. In some deposits, such as those at Blackpool, Littlejohns and Goonbarrow, the clay is ideally suited for the filling and coating of paper, while in other deposits, such as Treviscoe and Wheal Remfry, the clay is best suited for the making of ceramics. The manufacture of alum, sulphate of alumina and ultramarine uses up large quantities annually, besides which small quantities are used by manufacturing and colour makers, photographers and others. It has also been used, not avowedly however, as body in the composition of some artificial manure, the utilisation of sewage and even in the adulteration of flour. Glass, the preferred raw material for introducing alumina to glass compositions for glass fibre manufacture is kaolin.
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